The performance requirements for a filter cloth for air bags are as follows.
1 The woven fabric, from which an air bag is formed, should have a relatively high gas permeability and, when an automobile or aircraft collides and an inflation gas is introduced into the air bag, the air bag can allow a portion of the inflation gas to substantially evenly flow out so as to lower the gas temperature, to thereby prevent burning of an occupant by the inflation gas introduced into the air bag.
2 The filter cloth should appropriately filter a large mount of the fine particles contained in the inflation gas and maintain the air in the inside of the automobile or aircraft clean even after the inflation of the air bag.
3 After inflation, the air bag should be quickly deflated by discharging the inflation gas so as to not constrain the occupant.
As filter cloth for air bags meeting the above requirements, the following prior arts are been known. U.S. Pat. No. 4,840,397 for Katz et al. discloses an air bag device for a motorcar provided with a filter for cleaning an inflation gas. The filter is formed from a filter cloth which consists of a 2/1 twill weave made from aramid fiber spun yarns. This woven fabric has a single weaving structure and exhibits a high gas-permeability.
However, this filter cloth exhibits a poor resistance to seam slippage due to the single weaving structure having a high gas-permeability. Thus, when an inflation gas is blown into an air bag made from the filter cloth, the inflation gas can be discharged only through very small gaps formed between weaving yarns and distributed throughout the filter cloth. Accordingly, the filter cloth exhibits an unsatisfactory gas-discharging property and thus the air bag exhibits an unsatisfactory deflating property.
Japanese Unexamined Patent Publication No. 3-16,853 discloses a hollow weave fabric for air bags which is provided with specific gas-discharging segments having a mock leno weave structure.
However, in the air bag, since the mock leno weave structure is formed in vent hole portions (completely opening portions) specified in the hollow weave segment structure, the resultant air bag exhibits a poor deflating property after the introduction of the inflation gas. Also, since the mock leno weave structure is formed in specific portions of the woven fabric the air bag which can be produced from the woven fabric is limited to a specific type of air bags having a specific form.
European Unexamined Patent Publication (EP-A-) No. 442,373 (which corresponds to Japanese Unexamined Patent Publication No. 4-214,437) discloses a filter cloth consisting of a gas-discharging fabric. This filter cloth per se is a woven fabric having a single weave structure. Accordingly, this filter cloth has a poor resistance to seam slippage, similar to that of the filter cloth disclosed in the above-mentioned U.S. Patent. Also, when an inflation gas is blown into the air bag made from the filter cloth, the inflation gas can be discharged only through very small gaps formed between the weaving yarns and distributed throughout the air bag, and thus the air bag exhibits an unsatisfactory gas-discharging property and thus a poor deflating property.
Japanese Unexamined Patent Publication No. 4-262,938 discloses a filter cloth made from stretch-broken, fascinated yarns. However, this cloth consists of a woven fabric having a single weave structure similar to those disclosed in the above-mentioned prior arts and thus has the similar disadvantages to those mentioned above.
Finally, German Unexamined Patent Publication (DE-A-) No. 4126709 corresponding to U.S. Pat. No. 5,131,434 and to Japanese Unexamined Patent Publication No. 4-281,038 discloses a filter cloth having a plurality of segments different in weave structure and thus in gas permeability from each other. This cloth has weave structure segments having a high gas-permeability and thus a moderate filtering property, and other weave structure segments having a moderate or low gas-permeability and suitable for sewing. Namely, in this filter cloth, two or more segments different in function and in weave structure from each other are arranged in an appropriate layout for cutting of the cloth for the air bag.
According to the above-mentioned German patent publication specification, the segments having a high cutting property and a high sewing property for air bags are formed by a high density weave structure having a low gas-permeability and a high seam slippage resistance; other segments are formed by a moderate density weave structure having a moderate gas-permeability and still other segments having a high filtering property are formed by a low density weave structure having a high gas-permeability.
However, this type of filter cloth is disadvantageous in that the high cutting property, high sewing property segments are necessarily formed over both the high density weave structure portions and the moderate density weave structure portions and the moderate density weave structure portions exhibit a poor seam slippage resistance. Also this filter cloth has the commercial problem that since it is designed and woven in specific weaving structures suitable for an air bag of specific form and dimensions, the resultant filter cloth can be employed only for a specific type of air bag having a specific form and dimensions. The abovementioned German patent publication specification includes no description relating to the deflating property of the air bag after inflating.
Further, the high gas-permeability portions, which are so-called windows, formed in the filter cloth for an air bag, are woven in a 2/2 Panama weave structure to dimensions of a width of 40 cm and a length of 30 cm. Therefore, the window portions have a weave structure which exhibits a relatively high gas permeability compared to the ground plain weave structure. Namely, the window portions are not openings and thus must be formed over a relatively large area. The presence of the large area of the window portions (high gas-permeability portions) cause the freedom in trimming of the resultant filter cloth for cutting for the production of the air bag to be small.